The invention relates to a reaming tool for reaming a plurality of coaxial bores of a workpiece that are arranged at an axial distance from one another. The reaming tool includes a guide shank that can be clamped in place in a machine spindle and supported by external supporting elements in prepared bores of the workpiece to be machined and/or in a guide bush. The reaming tool includes a reaming head coaxially adjoining the front end of the guide shank and provided with cutting edges, and if need be, additional supporting elements.
To finish-ream camshaft or crankshaft bearing bores, a reaming tool with a long guide shank, which is supported in the pre-machined bores, is required in order to achieve the requisite alignment. In known reaming tools of this type, the reaming head is connected in one piece with the guide shank. A disadvantage there is that the rework of the cutting edges is relatively expensive. If the cutting edges are worn, the entire tool on the machine has to be exchanged.
Starting herefrom, the object of the invention is to develop a reaming tool having a guide shank of the type mentioned at the beginning, which reaming tool permits a simple change in the event of wear.
The features specified in claim 1 are proposed in order to achieve this object. Advantageous configurations and developments of the invention follow from the dependent claims.
According to a second embodiment variant of the invention, it is proposed, for the purpose of a kinematic reversal relative to the first embodiment variant, that the self-centering clamping mechanism have a tapered receptacle which is arranged concentrically in the reaming head and is defined by an annular flat face, a tapered spigot which projects coaxially forward beyond the guide shank, is defined by an annular flat face and can be inserted into the tapered receptacle, and a clamping screw which can be screwed from the reaming head into a coaxial internal thread of the guide shank, the reaming head and the guide shank, in the region of their flat faces, in the state free of stress, limiting a defined gage clearance dimension and, in the clamped state, bearing against one another with the tapered receptacle and the supporting elements arranged on the outside in this region being expanded elastically to a guide dimension. The rotary driving between the guide shank on the one hand and the reaming head on the other hand is expediently effected in this case by the reaming head having an internal hexagon arranged between tapered receptacle and annular flat face, while the guide shank has an external hexagon arranged between the tapered spigot and the annular flat face and complementary to the internal hexagon.
The clamping screw is expediently designed as a differential screw screwed into opposing internal threads of the reaming head and of the guide shank.
Tests have shown that the optimum gage clearance dimension lies between 0.01 and 0.06 mm, preferably 0.02 and 0.04 mm, while the taper angle of the tapered spigot and of the tapered receptacle should be 8xc2x0 to 16xc2x0. In order to avoid distortions in the region of the plane-parallel contact, it is important that the clamping screw, in the clamping state, is fastened with a tightening torque of D=X+d, in which case X denotes the tightening torque at the instant when the flat faces run against one another and d denotes an additional torque for producing a frictional connection in the order of magnitude of 0.4 to 4 Nm.
The supporting elements are advantageously designed as guide strips. A preferred configuration of the invention provides for the reaming head to have several, preferably four to six, cutting edges distributed over the circumference preferably at unequal spacing, and for the guide shank to have a corresponding number of guide strips distributed over the circumference and running in an axially parallel manner, the guide strips being arranged in the circumferential direction such as to be offset in each case from the spatially associated cutting edges. With these measures, a high surface quality in the bores to be machined can be achieved. A further improvement in this respect is achieved in that the flat face on the reaming-head side and the flat face on the rod side are continuous in the circumferential direction, that in other words no transverse grooves or transverse pins are arranged in this region.
A preferred method of producing a guide shank for the reaming tool according to the invention in accordance with the first embodiment variant provides for the pre-machined guide shank, provided with supporting elements or guide strips and with a front-end tapered receptacle, to be slipped onto a grinding arbor provided with a tapered spigot and a flat face defining the tapered spigot in an annular manner and to be clamped in place on this grinding arbor by means of a central clamping mechanism with the rod-side supporting elements or guide strips being partly expanded radially, and to then be ground on the outside diameter, with a predetermined diameter guide dimension being set, and to be removed again from the grinding arbor. The guide shank to be ground on the outside diameter, with a predetermined gage clearance dimension of preferably 0.01 to 0.06 mm being maintained, is put onto the tapered spigot of the grinding arbor and is tightened up to the stop with a defined clamping force against its flat face. To this end, the guide shank is advantageously clamped in place on the tapered spigot of the grinding arbor with a defined tightening torque by means of a differential screw.
Alternatively, to produce a reaming head for the reaming tool according to the invention in accordance with the second embodiment variant, it is proposed that the pre-machined reaming head, provided with supporting elements or guide strips and with a rear tapered receptacle, is slipped onto a grinding arbor provided with a tapered spigot and a flat face defining the tapered spigot in an annular manner and is clamped in place on this grinding arbor by means of a central clamping mechanism with the reaming-head-side supporting elements or guide strips being partly expanded radially, and is then ground on the outside diameter, with a predetermined diameter guide dimension being set, and is removed again from the grinding arbor. In this case, the reaming head to be ground on the outside diameter, with a predetermined gage clearance dimension of preferably 0.01 to 0.06 mm expediently being maintained, is put onto the tapered spigot of the grinding arbor and is tightened up to the stop with a defined clamping force against its fiat face. To this end, the reaming head is advantageously clamped in place on the tapered spigot of the grinding arbor with a defined tightening torque by means of a differential screw. The best repetitive accuracy is achieved if the differential screw, during the clamping operation, is fastened with a tightening torque D=X+d, in which case X denotes the tightening torque at the instant when the flat faces run against one another and d denotes an additional torque of 0.4 to 4 Nm for producing a frictional connection.